Methods of forming a variable watt density layered heater

ABSTRACT

Methods of forming a layered heater are provided that comprise at least one resistive layer comprising a resistive circuit pattern, the resistive circuit pattern defining a length, a thickness, and a spacing, wherein the thickness varies along the length of the trace of the resistive circuit pattern for a variable watt density. The methods include achieving the variable thickness by varying a dispensing rate of a conductive ink used to form the resistive circuit pattern, varying the feed rate of a target surface relative to the dispensing of the ink, and overwriting a volume of conductive ink on top of a previously formed trace of the resistive circuit pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/797,259, filed on Mar. 10, 2004 now U.S. Pat. No. 7,132,628, andtitled “Variable Watt Density Layered Heater.” The disclosure of theabove application is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to electrical heaters and moreparticularly to methods of forming an electrical heater for distributingwatt density.

BACKGROUND OF THE INVENTION

Layered heaters are typically used in applications where space islimited, when heat output needs vary across a surface, where rapidthermal response is desirous, or in ultra-clean applications wheremoisture or other contaminants can migrate into conventional heaters. Alayered heater generally comprises layers of different materials,namely, a dielectric and a resistive material, which are applied to asubstrate. The dielectric material is applied first to the substrate andprovides electrical isolation between the substrate and theelectrically-live resistive material and also reduces current leakage toground during operation. The resistive material is applied to thedielectric material in a predetermined pattern and provides a resistiveheater circuit. The layered heater also includes leads that connect theresistive heater circuit to an electrical power source, which istypically cycled by a temperature controller. The lead-to-resistivecircuit interface is also typically protected both mechanically andelectrically from extraneous contact by providing strain relief andelectrical isolation through a protective layer. Accordingly, layeredheaters are highly customizable for a variety of heating applications.

Layered heaters may be “thick” film, “thin” film, or “thermallysprayed,” among others, wherein the primary difference between thesetypes of layered heaters is the method in which the layers are formed.For example, the layers for thick film heaters are typically formedusing processes such as screen printing, decal application, or filmdispensing heads, among others. The layers for thin film heaters aretypically formed using deposition processes such as ion plating,sputtering, chemical vapor deposition (CVD), and physical vapordeposition (PVD), among others. Yet another series of processes distinctfrom thin and thick film techniques are those known as thermal sprayingprocesses, which may include by way of example flame spraying, plasmaspraying, wire arc spraying, and HVOF (High Velocity Oxygen Fuel), amongothers.

In some electrical heater applications, it may be desirable to vary thewatt density of the heater in certain areas in order to tailor theamount of heat delivered to the specific part or device being heated orto account for inherent variations in heat distribution along the heatertrace or element. Known electrical heaters typically vary the spacing ofthe resistive circuit pattern such that where the spacing is smaller andthe trace of the resistive circuit pattern is closer, the watt densityis higher, for a series circuit configuration. Conversely, the largerthe spacing between the traces of the resistive circuit pattern, thelower the watt density in those regions. In other known electricalheaters, the width of the trace of the resistive circuit pattern isvaried along its length in order to vary the watt density, wherein thewider the trace the lower the watt density and the narrower the tracethe higher the watt density, again, for a series circuit configuration.

SUMMARY OF THE INVENTION

In one preferred form, the present invention provides a layered heatercomprising at least one resistive layer comprising a resistive circuitpattern, wherein the resistive circuit pattern defines a length and athickness. The thickness of the resistive circuit pattern varies alongthe length of the resistive circuit pattern for a variable watt density.In other forms, both the thickness and a width of the resistive circuitpattern are varied, the thickness and a spacing are varied, or thethickness and the width and the spacing are varied for a variable wattdensity.

In another form, a layered heater is provided that comprises adielectric layer, a resistive layer formed on the dielectric layer, anda protective layer formed on the resistive layer. The resistive layercomprises a resistive circuit pattern, and the resistive circuit patterndefines a length and a thickness, wherein the thickness of the resistivecircuit pattern varies along the length of the resistive circuit patternfor a variable watt density. In another form, the dielectric layer isformed on a substrate.

In yet another form, a resistive circuit pattern for use in a layeredheater is provided, wherein the resistive circuit pattern defines alength and a variable thickness along the length of the resistivecircuit pattern.

According to methods of the present invention, resistive circuitpatterns of a layered heater are formed through the steps of dispensinga conductive ink at a rate onto a surface and varying the dispensingrate of the conductive ink to form a variable thickness resistivecircuit pattern. In another method, a feed speed of the surface relativeto the dispensing of conductive ink is varied to form a variablethickness resistive circuit pattern. Additionally, a method that variesthe dispensing rate and varies the feed speed of the surface relative tothe dispensing of conductive ink is provided in another form of thepresent invention.

Yet another method is provided that forms a resistive circuit pattern ofa layered heater through the steps of applying a volume of conductiveink onto a substrate to form a trace and applying an additional volumeof conductive ink onto the trace, wherein a variable thickness resistivecircuit pattern is produced.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a plan view of a layered heater system with a resistivecircuit pattern having variable spacing in accordance with a prior artheater system;

FIG. 2 is a plan view of a layered heater system with a resistivecircuit pattern having variable width in accordance with a prior artheater system;

FIG. 3 is a plan view of a layered heater system constructed inaccordance with the principles of the present invention;

FIG. 4 is a cross-sectional view, taken along line A-A of FIG. 3 androtated 90°, of a layered heater system with a resistive circuit patternhaving a variable thickness in accordance with the principles of thepresent invention;

FIG. 5 is a cross-sectional view, taken along line B-B of FIG. 3, of alayered heater system with a resistive circuit pattern having a variablethickness in accordance with the principles of the present invention;

FIG. 6 is a cross-sectional view, taken along line C-C of FIG. 3 androtated 90°, of a layered heater system with a resistive circuit patternhaving a variable thickness in accordance with the principles of thepresent invention;

FIG. 7 is a cross-sectional view, taken along line D-D of FIG. 3 androtated 90°, of a layered heater system with a resistive circuit patternhaving a variable thickness across a width in accordance with theprinciples of the present invention;

FIG. 8 is a plan view of another embodiment of a layered heater systemhaving a parallel circuit configuration and constructed in accordancewith the principles of the present invention;

FIG. 9 is a cross-sectional view, taken along line E-E of FIG. 8 androtated 90°, of a layered heater system with a parallel resistivecircuit pattern having a variable thickness in accordance with theprinciples of the present invention;

FIG. 10 is a cross-sectional view of resistive traces illustrating thedifferent watt densities for a series circuit configuration versus aparallel circuit configuration in accordance with the principles of thepresent invention;

FIG. 11 is a plan view of yet another embodiment of a layered heatersystem having a parallel-series-parallel circuit configuration andconstructed in accordance with the principles of the present invention;

FIG. 12 is a cross-sectional view, taken along line F-F of FIG. 11 androtated 90°, of a layered heater system with a parallel-series-parallelresistive circuit pattern, with a superimposed plot of watt density,having a variable thickness in accordance with the principles of thepresent invention;

FIG. 13 is a cross-sectional view of a layered heater system with aresistive circuit pattern having a variable thickness and a variablewidth in accordance with the principles of the present invention;

FIG. 14 is a cross-sectional view of a layered heater system with aresistive circuit pattern having a variable thickness and a variablespacing in accordance with the principles of the present invention;

FIG. 15 is a cross-sectional view of a layered heater system with aresistive circuit pattern having a variable thickness, a variable width,and a variable spacing in accordance with the principles of the presentinvention;

FIG. 16 is an elevated side view of a high coverage resistive circuitpattern in accordance with the principles of the present invention;

FIG. 17 is an enlarged cross-sectional view, taken along line G-G ofFIG. 16 and rotated 90°, of a variable watt density resistive circuitpattern in accordance with the principles of the present invention;

FIG. 18 is a cross-sectional view along the length of a constantthickness resistive circuit pattern having variable watt density inaccordance with the principles of the present invention;

FIG. 19 a is a cross-sectional view along the length of a continuousresistive circuit pattern in accordance with the principles of thepresent invention;

FIG. 19 b is a cross-sectional view along the length of a non-continuousresistive circuit pattern in accordance with the principles of thepresent invention;

FIG. 20 is a cross-sectional view of a layered heater system with aplurality of resistive layers, wherein the resistive layers compriseresistive circuit patterns that have a variable thickness in accordancewith the principles of the present invention; and

FIG. 21 is a cross-sectional view illustrating a method of forming avariable thickness resistive circuit pattern by overwriting a previouslyformed trace of the resistive circuit pattern in accordance with theprinciples of the present invention.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

Referring to FIGS. 1 and 2, two (2) prior art heater systems 10 and 12are illustrated that provide variable watt density. Both of the priorart heater systems 10 and 12 comprise resistive circuit patterns, 14 and16 respectively, which provide the requisite heating to the part ordevice to be heated. Generally, the resistive circuit pattern 14 in FIG.1 is formed on a substrate 15 and comprises a variable spacing (e.g., S1and S2) as shown in order to provide a variable watt density asrequired. In the areas of S1, the spacing is closer and thus the wattdensity is higher. Conversely, in the areas of S2, the spacing is widerand thus the watt density is lower.

As further shown in FIG. 2, the resistive circuit pattern 16 is formedon a substrate 17 and comprises a variable width (e.g., W1 and W2) inorder to provide a variable watt density as required. In the areas ofW1, the width is greater and thus the watt density is lower, whereas inthe areas of W2, the width is narrower and the watt density is higher.Accordingly, these prior art heater systems 10 and 12 employ a variablespacing or a variable width in order to vary the watt density asrequired.

More specifically, the watt density is a result of both a trace wattdensity, which is the watt density along the length or trace of theresistive circuit pattern (14, 16), and a substrate watt density, whichis the amount of coverage, or percent of the total substrate surfacearea that is covered by the resistive circuit pattern (14, 16), of theresistive circuit pattern (14, 16) across the entire substrate (15, 17).The trace watt density comprises the power being dissipated as afunction of the individual trace area, which, as used herein isgenerally defined by the width times the overall length of the resistivetrace. The substrate watt density comprises the power being dissipatedalong the resistive circuit pattern (14, 16) and the power beingdissipated as a function of the amount of coverage that the resistivecircuit pattern (14, 16) provides over the substrate (15, 17). Thus, thetrace watt density in FIG. 1 is constant across the entire substrate 15since the width of the resistive circuit pattern 14 is constant.However, since the spacing of the resistive circuit pattern 14 isvariable, the amount of coverage of the resistive circuit pattern 14over the substrate 15 varies, which results in a variable watt density.In FIG. 2, the trace watt density varies since the width of theresistive circuit pattern 14 varies, while the amount of coverage of theresistive circuit pattern 16 over the substrate 17 remains constant,which results in a variable watt density. Therefore, a variable wattdensity is achieved through a variable trace watt density and/or avariable coverage of the resistive circuit pattern (14, 16) over thesubstrate (15, 17). Accordingly, as used herein the term “watt density”should be construed to include either trace watt density or substratewatt density. As used herein, the term “coverage” should be construed tomean the total area of the resistive circuit pattern as compared with,or as a percentage of, the total area of a substrate. Accordingly, thehigher the total area of the resistive circuit pattern over a givensubstrate area, the higher the “coverage.”

Referring now to FIGS. 3 and 4, a layered heater that provides enhanceddesign flexibility to achieve variable watt density in accordance withthe present invention is illustrated and generally indicated byreference numeral 20. Generally, the layered heater 20 comprises anumber of layers disposed on a substrate 22, wherein the substrate 22may be a separate element disposed proximate the part or device to beheated, or the substrate 22 may be the part or device itself. As bestshown in FIG. 4, the layers preferably comprise a dielectric layer 24, aresistive layer 26, and a protective layer 28. The dielectric layer 24provides electrical isolation between the substrate 22 and the resistivelayer 26 and is formed on the substrate 22 in a thickness commensuratewith the power output, applied voltage, intended applicationtemperature, or combinations thereof, of the layered heater 20. Theresistive layer 26 is formed on the dielectric layer 24 and provides aheater circuit for the layered heater 20, thereby providing the heat tothe substrate 22. The protective layer 28 is formed on the resistivelayer 26 and is preferably an insulator, however other materials such asan electrically or thermally conductive material may also be employedaccording to the requirements of a specific heating application whileremaining within the scope of the present invention.

As further shown, terminal pads 30 are preferably disposed on thedielectric layer 24 and are in contact with the resistive layer 26.Accordingly, electrical leads (not shown) are in contact with theterminal pads 30 and connect the resistive layer 26 to a power source(not shown). As further shown, the protective layer 28 is formed on theresistive layer 26 and is preferably a dielectric material forelectrical isolation and protection of the resistive layer 26 from theoperating environment. Additionally, the protective layer 28 may cover aportion of the terminal pads 30 so long as there remains sufficient areato promote an electrical connection with the power source.

As used herein, the term “layered heater” should be construed to includeheaters that comprise at least one functional layer (e.g., dielectriclayer 24, resistive layer 26, and protective layer 28, among others),wherein the layer is formed through application or accumulation of amaterial to a substrate or another layer using processes associated withthick film, thin film, thermal spraying, or sol-gel, among others. Theseprocesses are also referred to as “layered processes,” “layeringprocesses,” or “layered heater processes.” Such processes and functionallayers are described in greater detail in co-pending application titled“Combined Layering Technologies for Electric Heaters,” filed on Jan. 6,2004, which is commonly assigned with the present application and thecontents of which are incorporated herein by reference in theirentirety.

As further shown in FIG. 3, the resistive layer 26 defines a resistivecircuit pattern 40, which comprises a length (shown as the distancealong the resistive circuit pattern 40 between the terminal pads 30), awidth W, and a spacing S. As illustrated in FIG. 4, the resistivecircuit pattern 40 further comprises a variable thickness along thelength L as shown by way of example in the areas having a thickness T1,thickness T2, and thickness T3. As shown, thickness T1 is greater thanthickness T2, and thickness T2 is greater than T3. In this example, ahigher watt density is required in the area of T3 versus the areas of T2and T1, and a higher watt density is required in the area of T2 than T1,generally due to heat loss through the edges of the substrate 22 for atypical heater application. Accordingly, the thickness of the resistivecircuit pattern 40 is thinner in areas where a higher watt density isrequired (higher resistance, greater heat transfer) and thicker in areaswhere a lower watt density is required (lower resistance, less heattransfer) during operation of the layered heater 20. Therefore, as thethickness varies along the length of the resistive circuit pattern 40,the resistance of the resistive circuit pattern 40 is varied along itslength, which results in a variable trace watt density. Methods forproducing the variable thickness resistive circuit pattern 40 aredescribed in greater detail below.

Referring now to FIGS. 5 and 6, a thickness T4 is shown through a curvedor “racetrack” portion of the resistive circuit pattern 40, which isthicker than an adjacent area having a thickness T5 along a linearportion of the resistive circuit pattern 40. This racetrack portion hastraditionally included a wider resistive circuit pattern due to theinherent build-up of current or “current crowding” in these areas duringoperation. The “current crowding” will result in high trace watt densityin the region adjacent the inner portion of the racetrack, leading to ahigher operating temperature and subsequent reliability degradation. Tomaintain a constant voltage, the pattern in prior art heaters has beendesigned to be wider to reduce the resistance where a current increaseoccurs. Unfortunately, this wider racetrack consumes additional spaceand to some degree dictates the spacing between the resistive circuitpattern 40 along the linear portions. The present invention overcomesthis disadvantage by increasing the thickness T4 along the racetrackportion rather than increasing the width of the resistive circuitpattern 40 such that additional space is not consumed and a more compactresistive circuit pattern 40 is provided.

In another form of the present invention as shown in FIG. 7, theresistive circuit pattern 40 comprises a variable thickness across thewidth of the racetrack portion. Along the inner portion of the racetrackwhere current crowding more specifically occurs, the resistive circuitpattern 40 comprises a thickness T6, which is thicker and has lessresistance than the outer portion of the racetrack that comprises athickness T7. As a result, the inner portion of the racetrack at T6 hasa lower watt density than the outer portion of the racetrack at T7 inorder to accommodate the current crowding, which promotes a more uniformtemperature throughout the entire racetrack portion. Therefore, thethickness varies across the width of the resistive circuit pattern 40from T6 to T7 in order to provide a variable watt density. It should beunderstood that the specific application of a variable thickness acrossthe width of the resistive circuit pattern 40 for a racetrackconfiguration is not intended to limit the scope of the presentinvention. The variable thickness across the width as illustrated anddescribed herein may be applied in any application where such variablewatt density is desired while remaining within the teachings of thepresent invention.

Referring to FIGS. 8 and 9, variable thickness according to another formof the present invention is employed in a parallel circuit configurationof a layered heater 42, rather than a series circuit configuration aspreviously described. As shown, a resistive circuit pattern 44 comprisesa series of resistive traces 46 that are connected to power busses 48.In a parallel circuit, assuming a constant voltage, the current in eachresistive trace 46 is a function of the resistance and is not constantas with the series circuit illustrated above. Since the power is equalto the square of the current times the resistance, (P=I²R), an increasein power is best achieved by an increase in current, which isaccomplished through an increase in thickness of the correspondingresistive trace 46. Therefore, in a similar application as describedabove where a higher watt density is required near the edges 45 and 47of the substrate 22, thickness T1′ is less than thickness T2′, andthickness T2′ is less than T3′. Accordingly, the thickness of theresistive circuit pattern 44 is thicker in areas where a higher wattdensity is required and thinner in areas where a lower watt density isrequired during operation of the layered heater 42 in a parallelcircuit. Therefore, as the thickness varies in each resistive trace 46of the resistive circuit pattern 44, the current varies within eachresistive trace 46, which results in a variable trace watt densityacross the substrate 22 (i.e., variable substrate watt density).

Therefore, depending on whether the resistive circuit pattern comprisesa series or a parallel circuit configuration, the thickness of theresistive trace varies differently. As illustrated in FIG. 10, for aparallel circuit configuration, the thickness of the resistive trace isthicker in areas where a higher watt density is required and thinnerwhere a lower watt density is required. Conversely, for a series circuitconfiguration, the thickness of the resistive trace is thicker where alower watt density is required and thinner where a higher watt densityis required. Accordingly, the circuit configuration, whether series orparallel, dictates whether the thickness should be increased ordecreased according to watt density requirements of the specificapplication.

In addition to the individual series and parallel circuits as describedabove, another form of the present invention comprises aparallel-series-parallel circuit as illustrated in FIGS. 11 and 12,wherein a layered heater 48 comprises a series circuit 49 in parallelwith parallel circuits 50. As shown, a resistive circuit pattern 51comprises parallel traces 52 and a series trace 53, which are connectedthrough power terminals 61 and power busses 63. In an application wherea higher watt density is required near edges 54 and 55 of the substrate22, the thickness of the resistive circuit pattern 51 near the edgeswithin the parallel traces 52 is thicker for a higher watt density, andthe thickness of the resistive circuit pattern 51 within the seriestrace 53 is also thicker for a lower watt density. Accordingly, bothparallel and series circuits may be combined with the variable thicknessaccording to the teachings of the present invention to achieve thedesired distribution of watt density. Therefore, the term“series-parallel” or “parallel-series” should be construed to mean acircuit that includes one or more series and parallel circuits withinthe same power circuit, regardless of the order of each of the seriesand parallel circuits within the power circuit.

As further shown in FIG. 12, a plot of the watt density across thesubstrate 22 is superimposed above the parallel traces 52 and the seriestrace 53 to further illustrate the different effect of variablethickness based on the type of circuit, i.e., parallel or series. As thethickness decreases across the parallel traces 52, the watt densitycorrespondingly decreases, however, as the thickness increases acrossthe series trace 53, the watt density continues to decrease. Therefore,the magnitude and direction (i.e., increase or decrease in thickness) ofvariable thickness according to the teachings of the present inventiondepends on the circuit configuration and the desired watt density acrossthe substrate 22.

In addition to varying the thickness of the resistive circuit pattern40, the width and/or the spacing may also be varied for additionaldesign flexibility in achieving a desired distribution of watt densityacross and along the substrate. Accordingly, FIG. 13 illustrates aresistive circuit pattern 57 having both a variable thickness (T8 andT9) and a variable width (W3 and W4), with a constant spacing in aseries circuit configuration. In the areas of T8 and W4, the resistivecircuit pattern 57 is relatively thin and narrow, whereas in the areasof T9 and W3, the resistive circuit pattern 57 is relatively thick andwide. As a result, a higher watt density is provided in the areas of T8and W4, and a lower watt density is provided in the areas of T9 and W3.

Referring to FIG. 14, a resistive circuit pattern 58 is illustrated inanother form of the present invention comprising both a variablethickness (T10 and T11) and a variable spacing (S3 and S4), with aconstant width in a series circuit configuration. As shown, in the areasof T10 and S3, the resistive circuit pattern 58 is relatively thin andhas closer spacing, whereas in the areas of T11 and S4, the resistivecircuit pattern 58 is relatively thick and has wider spacing. As aresult, a higher watt density is provided in the areas of T10 and S3,and a lower watt density is provided in the areas of T11 and S4.

Yet another form of the present invention is illustrated in FIG. 15,wherein a resistive circuit pattern 59 comprises a variable thickness(T12 and T13), a variable width (W5 and W6), and a variable spacing (S5and S6) in a series circuit configuration. In the areas of T12, W5, andS5, the resistive circuit pattern 59 is relatively thin, narrow, and hascloser spacing. In the areas of T13, W6, and S6, the resistive circuitpattern 59 is relatively thick, wide, and has wider spacing. Thus, ahigher watt density is provided in the areas of T12, W5, and S5, whereasa lower watt density is provided in the areas of T13, W6, and S6. Itshould be understood that the combinations of a variable thickness and avariable width, a variable thickness and a variable spacing, and avariable thickness, a variable width, and a variable spacing asdescribed herein may also be applied in a parallel circuit configurationor a series-parallel circuit configuration while remaining within thescope of the present invention.

FIG. 16 illustrates another form of the present invention, wherein aresistive circuit pattern 60 has a relatively high amount of coverageover the substrate 62. Such an application may exist where the size ofthe substrate 62 is limited due to a relatively small target part ordevice being heated, or where space is limited for the layered heaterwithin the specific application. An example of an application where highwatt density is required in a relatively compact size is a quartz tubeheater in chemical applications, which is illustrated and generallyindicated by reference numeral 64. Generally, a chemical solution entersan inlet port 66 at a proximal end 68, flows through the quartz tubeheater 64 to be heated, and then flows through an outlet port 70 at adistal end 72. Thus, the chemical solution is at a lower temperature atthe inlet port 66 than it is at the outlet port 70, which results in arelatively wide temperature distribution through the quartz tube heater64. To create a more uniform temperature distribution, the watt densityat the proximal end 68 should be higher than the watt density at thedistal end 72. However, since the coverage of the resistive circuitpattern 60 is relatively high, there is little to no room to tailor thespacing or the width of the resistive circuit pattern 60. Therefore, thethickness of the resistive circuit pattern 60 is thinner at the proximalend 68 and is thicker at the distal end 72 in accordance with one formof the present invention as shown in FIG. 17, which results in avariable watt density resistive circuit pattern 60.

In addition to a variable thickness and other variable geometries aspreviously described, another form of the present invention provides avariable watt density through a change in composition of the resistivecircuit pattern material along its length. As shown in FIG. 18, aresistive circuit pattern 80 defines a constant thickness T14 along itslength. However, the material composition of the resistive circuitpattern 80 changes along its length, for example, transitioning from ahigher resistance composition at portion 82 and a lower resistancecomposition at portion 84. As a result, the watt density is higher atportion 82 than the watt density at portion 84, resulting in a variablewatt density resistive circuit pattern 80. Therefore, varying thematerial composition of the resistive circuit pattern 80 providesadditional design flexibility in providing a variable watt densitylayered heater in accordance with another form of the present invention.It should be understood that such a variation in material compositionmay be combined with the variable thickness and other variablegeometries while remaining within the scope of the present invention.

In yet another form of the present invention, the variable thickness andother geometries as previously described may be either continuous asshown in FIG. 19 a or non-continuous as shown in FIG. 19 b. A continuousresistive circuit pattern 90, which is formed by processes that aredescribed in greater detail below, defines a gradual change in thicknessfrom T15 to T16. As a result, a gradual decrease in watt density occursalong the length of the resistive circuit pattern 90 from T15 to T16.Alternately, a non-continuous resistive circuit pattern 92 may beproduced that defines a step change in thickness from T17 to T18 to T19.Accordingly, a step change in the watt density from high to low resultsalong the length of the resistive circuit pattern 92 from T17 to T19.Processes for the non-continuous resistive circuit pattern 92 are alsodescribed in greater detail below. These continuous and non-continuousconfigurations may be applied to not only to the thickness asillustrated herein, but also to the width of the resistive circuitpatterns while remaining within the scope of the present invention.

Referring to FIG. 20, another form of the present invention includes alayered heater 110 that comprises a plurality of resistive layers 112and 114, wherein each of the resistive layers 112 and 114 defines atleast one resistive circuit pattern with variable thickness resistivetraces 116 and 118, respectively. For the purposes of this embodiment,the circuit configuration is series, although it should be understoodthat parallel and/or series-parallel circuit configurations may also beemployed within the plurality of resistive layers. Accordingly, inapplications where the substrate 120 defines an insufficient surfacearea to accommodate a resistive circuit pattern to produce the requiredwatt density on a single layer, a plurality of resistive circuitpatterns are employed in a plurality of layers as shown. Additionally,the thickness is varied as shown according to watt density requirements,wherein a higher watt density is required near edges 122 and 124 of thesubstrate 120. Therefore, variable thickness according to the teachingsof the present invention is employed within multiple resistive layers toprovide the required watt density when the substrate surface area islimited. It should also be understood that the thickness may vary acrossthe width in addition to along the length as shown and that a variablewidth and/or spacing may also be employed while remaining within thescope of the present invention.

Furthermore, it should be understood that the thicknesses of theresistive circuit patterns as shown and described herein are variedaccording to specific heater application requirements and theembodiments illustrated above are exemplary only and should not beconstrued as limiting the scope of the present invention. Accordingly,different patterns with different areas and configurations of varyingthicknesses, along with varying width and or spacing, within seriesand/or parallel circuits, and disposed within a plurality of resistivelayers, among other configurations, may be also employed while remainingwithin the scope of the present invention.

According to one method of the present invention, the variable wattdensity resistive circuit patterns as described herein are formed byvarying the rate at which an electrically conductive ink is dispensedonto a surface, e.g., onto the dielectric layer 24. The conductive inkmay be dispensed using precision pen writing equipment, which isdescribed in greater detail in U.S. Pat. No. 5,973,296 and commonlyassigned with the present application, the contents of which areincorporated herein by reference in its entirety. With the precision penwriting equipment, the conductive ink is dispensed through an orifice ina writing tip while the tip and/or the target substrate is translated inorder to produce a predetermined resistive circuit pattern. In order toachieve a variable thickness where desired, the rate at which theconductive ink is dispensed, or the flow rate of the electricallyconductive ink through the tip orifice, is varied. For example, in areaswhere a thicker resistive circuit pattern is desired, the rate at whichthe conductive ink is dispensed is increased. Conversely, in areas wherea thinner resistive circuit pattern is desired, the rate at which theconductive ink is dispensed is reduced. Accordingly, a variablethickness resistive circuit pattern is produced by varying the rate atwhich the conductive ink is dispensed onto the target surface.

According to another method of the present invention, the rate at whichthe target surface is moved relative to the writing pen (i.e., feedspeed) is varied in order to produce a variable thickness resistivecircuit pattern. In areas where a thicker resistive circuit pattern isdesired, the feed speed of the target surface is reduced relative to thewriting pen while the rate at which the conductive ink is dispensedremains constant. Alternately, in areas where a thinner resistivecircuit pattern is desired, the feed speed of the target surface isincreased relative to the writing pen, again while the rate at which theconductive ink is dispensed remains constant. In yet another form of thepresent invention, both the dispensing rate of the conductive ink andthe feed speed of the target surface may be varied in order to produce avariable thickness resistive circuit pattern. Both the dispensing rateand the feed speed may be varied either continuously or non-continuouslyto produce the continuous and non-continuous resistive circuit patternsas previously described while remaining within the scope of the presentinvention.

In yet another method as shown in FIG. 21, the present inventionproduces a variable thickness resistive circuit pattern by overwriting avolume of conductive ink on top of a previously formed trace of theresistive circuit pattern. More specifically, the method includes thesteps of dispensing a volume of conductive ink from a pen 94 onto asurface 96 to form a trace 98 and then selectively dispensing anadditional volume 100 of conductive ink onto the previously formed trace98, wherein a variable thickness resistive circuit pattern is produced.Therefore, in areas where a thicker resistive circuit pattern isdesired, an additional volume 100 of conductive ink is formed over apreviously formed trace 98 in accordance with the principles of thepresent invention. Furthermore, either the pen 94 and/or the surface 96are moved relative to one another during the dispensing of theconductive ink to form the desired resistive circuit pattern.Alternately, the volumes of conductive ink may be applied using processother than the precision pen writing equipment while remaining withinthe scope of the present invention. Other layering processes associatedwith thick film, thin film, thermal spraying, or sol-gel may be used toapply the volumes of conductive ink. For example, the thick film processof silk-screening may be employed to apply the volumes of conductive inkin one alternate form of the present invention. It should be understoodthat application of the additional volumes of conductive ink is notlimited to a sequential application after the original volume is appliedwithin a manufacturing process. More specifically, the additional volumemay be applied at any stage of the manufacturing process such as, by wayof example, after a drying operation or after a firing operation.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. For example, theheater systems as described herein may be employed with a two-wirecontroller as shown and described in co-pending application Ser. No.10/719,327, titled “Two-Wire Layered Heater System,” filed Nov. 21,2003, and in co-pending applications titled “Combined Material LayeringTechnologies for Electric Heaters,” and “Tailored Heat Transfer LayeredHeater System,” both filed Jan. 6, 2004, and all of which are commonlyassigned with the present application and the contents of which areincorporated herein by reference in their entirety. Further, thecross-sectional profile of the resistive circuit pattern is not limitedto rectangular shapes as illustrated herein. The leveling qualities ofthe ink may not produce such a shape after processing, and other shapesmay be desired such as the variable thickness width through theracetrack portions as previously described and illustrated.Additionally, the resistive circuit patterns herein are illustrated onrelatively flat and rectangular substrates for purposes of clarity, andit should be understood that other substrate geometries such ascylinders and other 3D shapes, such as those illustrated with the quartzheater embodiment, are within the scope of the present invention.Moreover, the circuits as illustrated herein are in series or inparallel, and it should be understood that the various embodiments ofthe present invention may also be employed with series-parallel circuitswhile remaining within the scope of the present invention. Suchvariations are not to be regarded as a departure from the spirit andscope of the invention.

1. A method of forming a trace of a resistive circuit pattern of alayered heater, the trace defining a length, a width, and a spacing, themethod comprising the steps of: (a) dispensing a conductive ink throughan orifice of a single writing tip at a rate onto a surface; and (b)selectively varying the dispensing rate of the conductive ink throughthe orifice of the writing tip to form a variable thickness trace of theresistive circuit pattern.
 2. A method of forming a trace of a resistivecircuit pattern of a layered heater, the method comprising the steps of:(a) dispensing a conductive ink through an orifice of a single writingtip at a rate onto a surface; and (b) selectively varying a feed speedof the substrate relative to the dispensing of conductive ink throughthe orifice of the writing tip to form a variable thickness trace of theresistive circuit pattern.
 3. A method of forming a trace of a resistivecircuit pattern of a layered heater, the method comprising the steps of:(a) applying a volume of conductive ink onto a surface with a singledispensing element to form the trace; and (b) subsequently applying anadditional volume of conductive ink onto the trace with the singledispensing element after the volume of step (a) has been applied alongthe entire trace, wherein a variable thickness trace of the resistivecircuit pattern is produced.
 4. The method according to claim 3, whereinthe volumes of conductive ink are applied by a layered process selectedfrom the group consisting of thick film, thin film, thermal spraying,and sol-gel.
 5. The method according to claim 3, wherein the volumes ofconductive ink are applied using precision pen writing equipment.
 6. Themethod according to claim 3, wherein the volumes of conductive ink areapplied using a silk screening process.
 7. A method of forming a traceof a resistive circuit pattern of a layered heater, the trace defining alength, a width, and a spacing, the method comprising forming the tracewith a variable thickness.
 8. The method according to claim 7, whereinthe variable thickness is formed along the length of the trace of theresistive circuit pattern.
 9. The method according to claim 7, whereinthe variable thickness is formed across the width of the trace of theresistive circuit pattern.
 10. The method according to claim 7, whereinthe trace of the resistive circuit pattern is formed by a processselected from the group consisting of thick film, thin film, thermalspraying, and sol-gel.
 11. The method according to claim 3, wherein thevolumes of conductive ink are applied using a thick film process. 12.The method according to claim 3, wherein the volumes of conductive inkare applied using a thin film process.
 13. The method according to claim3, wherein the volumes of conductive ink are applied using a thermalspray process.
 14. The method according to claim 3, wherein the volumesof conductive ink are applied using a sol-gel process.
 15. The methodaccording to claim 7, wherein the volumes of conductive ink are appliedusing a thick film process.
 16. The method according to claim 7, whereinthe volumes of conductive ink are applied using a thin film process. 17.The method according to claim 7, wherein the volumes of conductive inkare applied using a thermal spray process.
 18. The method according toclaim 7, wherein the volumes of conductive ink are applied using asol-gel process.
 19. The method according to claim 7, wherein the traceis formed with a variable thickness continuously.
 20. The methodaccording to claim 7, wherein the trace is formed with a variablethickness non-continuously.